1. Field of the Invention
The present invention relates to a method for multi-user MIMO transmission, more in particular, a method for transmission between a base station and U (>1) user terminals, said base station and user terminals each equipped with more than one antenna, preferably in conjunction with considering the optimizing of joint transmit and receive filters, for instance in a MMSE context. Further disclosed are base station and user terminal devices suited for execution of said method.
2. Description of the Related Technology
Multi-input multi-output (MIMO) wireless communications have attracted a lot of interest in the recent years as they offer a multiplicity of spatial channels for the radio links, hence provide a significant capacity or diversity increase compared to conventional single antenna communications.
Multi-Input Multi-Output (MIMO) wireless channels have significantly higher capacities than conventional Single-Input Single-Output (SISO) channels. These capacities are related to the multiple parallel spatial subchannels that are opened through the use of multiple antennas at both the transmitter and the receiver. Spatial Multiplexing (SM) is a technique that transmits parallel independent data-streams on these available spatial subchannels in an attempt to approach the MIMO capacities.
In addition, Spatial-Division Multiple Access (SDMA) is very appealing due to its inherent reuse (simultaneously for various users due to the exploitation of the distinct spatial signatures of the users) of the precious frequency bandwidth.
Several MIMO approaches can be followed which can be classified according to whether or not they require channel knowledge at either the transmitter or the receiver. Typically, the best performance can be obtained when the channel is known at both sides.
The optimal solution is provided by SVD weights combined with a water-pouring strategy. However, this strategy must adaptively control the number of streams and also the modulation and coding in each stream, which makes it inconvenient for wireless channels.
A sub-optimal approach consists of using a fixed number of data streams and identical modulation and coding as in a single-user joint transmit-receive (TX-RX) MMSE optimization [H. Sampath and A. Paulraj, “Joint TX & RX Optimization for High Data Rate Wireless Communication Using Multiple Antennas”, Asilomar conf. On signals, systems and computers, pp. 215-219, Asilomar, Calif., November 1999]. This latter solution is more convenient but is not directly applicable to SDMA MIMO communications where a multi-antenna base station communicates at the same time with several multi-antenna terminals. Indeed, the joint TX-RX optimization requires channel knowledge at both sides, which is rather unfeasible at the terminal side (the terminal only knows its part of the multi-user wireless channel).
To approach the potential MIMO capacity while optimizing the system performance, several joint TX/RX MMSE designs have been proposed.
Two main design trends have emerged that enable Spatial Multiplexing corresponding to whether Channel State Information (CSI) is available at the transmitter. On the one hand, BLAST-like space-time techniques make use of the available transmit antennas to transmit as many independent streams and do not require CSI at the transmitter. On the other hand, the joint transmit and receive space-time processing takes advantage of the potentially available CSI at both sides of the link to maximize the system's information rate or alternatively optimize the system performance, under a fixed rate constraint.
Within multi-user MIMO transmission schemes multi-user interference results in a performance limitation. Further, the joint determination of optimal filters for both the base station and the user terminals in the case of a multi-user context is very complex.